This invention relates generally to the processing of blown tube films and is particularly directed to processing of blown tube films in which the film undergoes a change in shape either from a generally circular cross-section to the layflat shape, or from the layflat shape to a generally circular cross-sectional shape. The invention will be specifically disclosed in connection with a method and apparatus which, through the approximation and equalization of circumferentially varying streamline lengths, substantially equalizes circumferentially varying shape change induced machine direction stresses, thereby reducing the residence time variations within higher thermal energy regions of streamline elements within those regions and the concomitant variations of material properties of the product issuing from the tension isolated region including significantly minimizing the commonly rather large magnitudes of variations of gauge, energy to break, and machine direction lengths of the solidified material. Further, through the minimization of the magnitude of variation of the circumferentially varying machine direction stresses and the gradients associated with those stresses, thereby improving the uniformity of the residence time of material elements within any higher thermal energy states of material elements which might occur within any tension isolated region of the process, there is offered, thereby, potential to structure more uniformly the macromolecular arrangements existing within the material issuing from the process plus allowing the blown film extrusion of higher modulus materials than have heretofore been possible.
The blown film process is well known. The process generally includes the continuous extrusion of molten polymeric material through an annular die opening, which is the beginning of the blown film process. The molten polymeric material exits the die opening at a diameter approximately equal to the diameter of the die opening. Most commonly, the diameter of the blown film tube then undergoes a significant increase as the material advances due to the circumferential component of pneumatic pressure exerted by air encapsulated within the film against the inside surface of the draw region of the blown film tube. Alternatively, the diameter increase may be effected by drawing the circular film over an internally disposed mandrel. Another alternative is that the interior surface of the tube may be left open to the atmospheric pressure and cooled such that the final diameter of solidified material is less than the diameter of the die.
The melted blown tube film undergoes cooling upon exiting the die opening, which may typically be enhanced by directing a flow of cooling air about the periphery of the tube. As the advancing tube expands in diameter, it reaches an equilibrium state at which the material solidifies sufficiently so as to prevent any further inelastic deformation thereof. The solidification of the blown tube film occurs in a known as the frost band. The region between the terminus of the frost band (i.e. the point at which the blown tube film is completely solidified about its circumference, and no longer deforms inelastically in any direction) and the die exit is known as the draw region, and the region downstream of the frost band terminus may be referred to as the solid material region.
In order to handle the continuously moving blown tube film, it is typically collapsed from its generally circular cross-sectional shape to what is know as the layflat shape in which its cross-sectional shape is essentially a straight line whose width is equal to one half the circumference of the tube. The blown tube film in the layflat configuration may be advanced by or around rollers and processed further immediately, or stored as a roll for further processing at a later time. Several different prior art collapsing geometries are described below for effecting the shape change of the blown tube film from circular to layflat.
One method of processing a previously collapsed blown tube film includes the expansion of the layflat film back to a circular cross-sectional shape for various purposes. For example, the layflat film may be expanded into the circular cross-section, heated so as to allow the diameter of the tube to increase further, cooled and then recollapsed into a wider layflat configuration.
There are several commonly acknowledged problems existent in blown film products or processes. One problem is that blown film has very large gauge, or thickness, variations. Blown film typically has randomly located gauge variations greater than tl5% of the mean gauge of the film. Such gauge variation has a notable undesirable effect on both quality and cost of products made from such blown film, as well as limits those products which can be made from blown film.
Another problem is the wide variation of strength of random samples of the same blown tube film. The strength of blown film is measured by a standard test generally referred to as the dart drop test. To conduct the test, a portion of the blown film is secured within a framework, and a standard dart is dropped onto the film from a specified height to analyze the energy necessary to break the blown film. The energy to break of different portions of blown tube film consistently yields even greater variations in energy to break results than the gauge thickness variations present in the blown film. Such energy to break variations result in constant quality problems in products manufactured from the blown film process.
Another problem is also manifested by blown film in the wound roll condition. It has been observed that wound rolls of blown film consistently exhibit xe2x80x9csoftnessxe2x80x9d at the center web of the roll, with a hard xe2x80x9cbandxe2x80x9d located a distance approximately 10% to 15% of the total width of the layflat roll from each edge. The edge regions themselves are softer than these hard band regions, although not as soft as the center region of the wound roll.
Additionally, certain problems occur in dependence upon the material being processed. In particular, high density polyethylene (HDPE) materials, which have a high modulus of elasticity in comparison to other materials such as low density polyethylene (LDPE), are well known for the difficulties which occur in attempting to utilize them in the blown film process. One example of such problems is the occurrence of excess sag in the center region of the layflat web. This sag makes post processing of HDPE blown film very difficult. When more than 4 or 5 slit-seal lanes are run from slit seal towers toward bag machines being operated in line with contemporaneously blown film extrusion when processing HDPE, time related center lane increasing sag coupled with contemporaneously increasing tension in the edge lanes cause increasing levels of operational difficulties as the number of slit-seal lanes are increased. Running of more than four lanes yields unfavorable production costs due to the increasing magnitude of and the operating control problems associated with this center sag, edge tension problem. Web sag, edge curl, and edge sag may also be present to a greater or lesser degree in processed blown film, depending upon the modulus of the material being run.
There is a processing problem which exists in the collapsing of the blown film tubes. The change of geometric shape from a circular cross-section to a layflat configuration, particularly in a continuously moving blown film, causes length variations of streamlines as the shape is changed to the layflat configuration. A streamline, as used in the specification and the claims herein, is the path followed by any differential element of the blown tube film as it is processed, for example, as it advances from the frost band terminus to the initial layflat configuration. The problem exists whenever the shape of the blown film is changed from circular to layflat, or from layflat to circular. In this patent, web shape change from either tubular to layflat shape or from layflat shape to tubular shape will be herein referred to as xe2x80x9ccollapsingxe2x80x9d, and the geometric shaping device accomplishing such shape change are herein referred to as the xe2x80x9ccollapserxe2x80x9d. Unequal streamline lengths have been identified in the prior art as causing inelastic stretching and bagging of the final film, thereby frequently producing unusable film, or severely limiting the uses which can be made of the film.
As mentioned above, there are several collapsing geometries known in the prior art for changing the generally circular cross-sectional shape of the blown tube film to the layflat configuration. As used herein, and in the claims, xe2x80x9ccollapsing geometryxe2x80x9d includes the structure which directly contacts the blown tube film so as to effect such shape change and the support structure thereof. One of the most frequently used collapsing geometries is known as an A-Frame collapser, which as most commonly adopted, has developed to a design which combines the features of two patents. The nip roll feature of U.S. Pat. No. 2,461,976 to B. H. Schenk, which discloses the use of large diameter nip rolls to both haul film away from the die and to collapse the tube shape to the layflat film shape, has, in current practice, been combined with the A-Frame portion of the collapsing arrangement disclosed in U.S. Pat. No. 2,529,897 to Bailey, et al. The A-Frame collapsing geometry includes two spaced apart converging plane surface planes which are parallel to each other at any height of the A-Frame section, and located adjacent the blown tube film so as to change its shape from circular cross-section to layflat. These two planes define the collapsing geometry boundary for A-Frame collapsing.
The completed layflat configuration may occur at the exit of the A-Frame collapser, where the spaced apart frames are closest to each other, or may occur adjacent nip rolls which are located downstream of the A-Frame collapser so as to advance the blown tube film. An A-Frame may also be used to effect the shape change of a blown tube film from layflat to circular cross-section.
U.S. Pat. No. 2,461,976 to Schenk describes what is referred to herein as nip roll collapsing. In this configuration, the change in shape of blown tube film occurs as the result of its contact with a pair of nip rolls located in the processing line downstream of the frost line. However, as described above in reference to A-Frame collapsing, nip rolls may be used with any other collapsing geometric.
Another method of collapsing the generally circular cross-sectional tube to the layflat configuration is shown in U.S. Pat. Nos. 2,720,680, 3,061,875, 3,144,494, and 3,304,352, all to Gerow. In this type of collapsing, identified herein as spreader collapsing, the shape change of the tube is effected by a spreader located inside of the tube which contacts the inner surface of the tube and xe2x80x9cspreadsxe2x80x9d it to a width approximating the layflat width. The closeness of the web to the completed layflat configuration depends upon the dimensions of the collapsing xe2x80x9carmsxe2x80x9d of the spreader collapser. The transformation to the layflat configuration is then completed by a pair of nip rollers located downstream of the spreader collapser.
U.S. Pat. No. 3,426,113 to Yazawa discloses a collapsing geometry which is referred to herein as unwrap flattening. In unwrap flattening, the continuously moving tube is slit along a streamline and the xe2x80x9copenxe2x80x9d tube is unwrapped and laid flat in a configuration which has a total width equal to the total circumference of the tube. The unwrapped film passes over a roller or a pair of nip rollers which are located on the center axis of the tube. An alternative method is to locate the roller or nip rollers offset from the central axis of the tube, which is herein referred to as unwrap offset flattening. A similar collapsing configuration is shown in U.S. Pat. No. 3,313,870 to Yazawa in which the tube is slit at locations spaced apart 180xc2x0 from each other, and the resulting two pieces of film are laid flat by passing them over rollers or between nip rollers which are not located on the axis of the tube. Such collapsing as identified in U.S. Pat. No. 3,313,870 is referred to herein as two unwrap offset flattening. All of the above-described collapser geometries, i.e., A-Frame collapsing, nip roll collapsing, spreader collapsing, unwrap flattening, unwrap offset flattening, two unwrap offset flattening, have inherent variations between the circumferentially positioned streamline lengths as a result of the shape change.
U.S. Pat. No. 4,170,624 to Dawson discloses a modification of the A-Frame collapsing geometry, referred to herein as xe2x80x9carticulated collapsingxe2x80x9d, which reduces, but does not eliminate, variations in streamline lengths of a tube undergoing flattening. Dawson utilizes the conclusion taught by U.S. Pat. No. 3,258,516 to Ewing, that the streamline lengths of a rectangular cross-sectional shaped tube being changed from one rectangular shape to any other rectangular shape are equal. Dawson discloses the addition of a second, truncated A-Frame collapser, which collapses opposed segments of the circular tube that are located between the two collapsing frames of the first A-Frame collapser. All four frames cooperate to collapse the circular tube into a rectangular shape, more specifically disclosed as a square cross-section. The disclosure in Dawson indicates that the collapsing of a tube from a circular cross-section to a flat cross-section causes uneven stresses to be imposed on or set up in the film in the collapsing region, producing stretching and bagging of the final film. However, articulated collapsing as disclosed by Dawson does nothing to minimize variations between streamline lengths which occur when the cross-sectional shape is changed from circular to square or rectangular xe2x80x9cboxedxe2x80x9d shapes, the sides of which are straight line segments and are perpendicular to the adjoining sides. Nor does the teaching of Dawson account for the nip rolls located downstream of the articulated collapser exit.
U.S. Pat. Nos. 2,461,976, 2,529,897, 2,720,680, 3,061,875, 3,144,494, 3,304,352, 3,426,113, 3,313,870 and 4,170,624 are incorporated herein by reference.
In general, while there has been a recognition that streamline length variations exist in blown tube films undergoing collapsing shape change between generally circular cross-sections to layflat cross-sections, the prior art has not correctly and properly identified the direct results of such length variations, as well as the mechanisms through which problems are thus created.
The prior art has virtually ignored the importance of the fact that within any tension isolated region of the processing of tubular circumferentially contiguous material, manufactured either contemporaneously (in-line) or non-contemporaneously (off-line) with film extrusion in the blown film process, where, within such tension isolated region, when the material is in a generally circular cross-sectional shape and under internal pneumatic pressure, and the material within the tension isolated region also undergoes one or more collapsings, there exist circumferentially varying collapsing streamline path lengths through each and every collapsing which may occur within that tension isolated region. The prior art has not recognized that these varying collapsing streamline lengths of the solidified material passing through the collapser induce into the material within the tension isolated region significantly large circumferentially varying collapsing induced machine direction stresses which are several orders of magnitude larger than any pneumatically induced machine direction stress which may exist as a result of the differential gauge pressure above atmospheric pressure of the air internally encapsulated by the contiguous web surface of that tension isolated region of the process. The length of any collapsing region streamline above the shortest collapsing streamline length which exists in that collapser is, in reality, a machine direction elongation of the solidified material web passing through that collapser. Further, the elongation of any solid streamline divided by the solidified length of that streamline is the elastic strain of that streamline induced by that collapser. The stress magnitude due to the strain of the solidified material of a solid streamline is proportional to the amount of elastic strain experienced by the solidified portion of the streamline material multiplied by the tensile modulus of the solidified material.
These circumferentially varying collapsing induced machine direction tension stresses (1-1 streamline direction collapsing stresses) in the web along each streamline, in turn, combine with the pneumatically induced circumferentially uniform machine direction tension stress (pneumatic pressure streamline direction (1-1 direction) stress) to draw the material away from whatever upstream tension isolating device exists at the start of the tension isolated segment. The upstream tension isolating device may be a nip roll or other similar web normal pressure tension isolating device, or, at the occasion of the start of the blown film process (i.e., at the blown film extruder) the machine direction tension isolating device is the blown film extrusion die efflux exit. Thus, the combined circumferentially varying collapsing induced machine direction stress plus the circumferentially uniform pneumatic pressure induced machine direction stress produce a circumferentially varying streamline direction combined stress pattern which is induced in the web material at any streamline all the way from the upstream tension isolating device at the start of any tension isolated region of any blown film process to the downstream tension isolating device of that tension isolated region.
Thermal energy cooling devices are normally positioned adjacent draw regions at the start of the blown film process, such as internal and/or external air rings and/or cooled mandrels, in order to cool web material temperatures within the draw region to values low enough so as to cause a change in the phase of the material from an elongationally drawable melt state at the start of the draw region to a solidified state at the terminus of the frost band of the draw region. Since there are material temperature gradients in the thickness normal direction (3-3 direction) of the web material as well as in the streamline direction (1-1 direction) of the web material within all of the draw region, then the collapsing induced circumferentially varying residence time of material differential elements within the draw region, and the gradients of those material residence times between adjacent streamline material elements within the draw region, coupled with the presence of solidification and orientation of solidifying structural elements of the material within the frost band region of the draw region of that tension isolated region of the process, all combine to cause deleterious material unit structure arrangements (orientations) within web materials, cause deleterious process effects, and, thereby, cause deleterious material properties effects within the products issuing from the start of the blown film process.
It is to be further noted that these deleterious collapsing effectxe2x80x94material structure interrelationships exist not only at the start of the blown film process, but also exist within any tension isolated region of blown film processing where both collapsing exists and sufficient material internal energy is added to the solid processing material from any external heat source to a degree sufficient as to allow either solid material ordered structure growth or realignment of relative positions of solid material ordered structure units to be influenced by the circumferentially varying machine direction stress pattern induced by collapsing. Both of these material structure changes result in collapsing influenced deleterious material structure orientations. Thus, even when materials are processed within a tension isolated region at temperatures below melt temperature, collapsing induces previously unrecognized detrimental orientations within the structure of the material issuing from that tension isolated region.
Examples of deleterious in-line process effects caused by collapsing induced problems are the processing difficulties normally encountered upon attempts at blown film extrusion of higher modulus polymeric materials, the circumferential direction (2-2 direction) variations of the surface temperatures of materials within draw regions of the process, web sag, wrinkles, edge curls, and in-line processing tension control difficulties associated with control of in-line downstream tension isolated regions when multiple lane slit configurations are obtained from single layflat master webs during in-line processing.
Examples of non-contemporaneous processing difficulties experienced due to collapsing effects in blown film materials are the difficulties encountered by slitting machine operators in controlling tension at individual slit lane winding spools during off-line slitting of blown film extruded webs, and the wrinkling and sag problems normally associated with blown film webs when they are run through off-line flexographic and rotogravure printing presses.
Examples of deleterious product effects in materials issuing from the blown film process are the hard and soft regions of roll stock issuing from the process, gauge variations, material energy to break variations, solidified machine direction length variations, and the low values of transverse direction modulus of blown film materials when compared to the machine direction modulus of the same web extruded from the blown film process. Further, with regard to impact testing, the impact failure direction of blown film extruded material when subjected to dart drop testing is, generally, anisotropic in nature in that regardless of the blow up ratio (blown film radius to die radius ratio) utilized at the start of blown film extrusion, the failure direction of the rupture of the film within blown film product materials when subjected to dart drop tests rather consistently propagate near to or directly in the streamline direction of blown film product materials.
Since the magnitude of the variations with each of these deleterious quality effects are associated with collapsing induced stress, and, since the stress associated with any given elongation amount is proportional to the tensile modulus of the material being processed, then materials of higher modulus than LDPE are to be expected to run with more difficulty through the blown film process, and, further, it is to be expected that the blown film processing of higher modulus materials will produce materials with greater magnitudes of variation of deleterious process and product problems than will blown film extrusion processing of LDPE.
The present invention includes a method for processing blown film which includes approximating the shape change induced machine direction stresses present at the frost band terminus and reducing them to nearly zero. The present invention also includes a method for approximating the reference streamline path lengths of circumferentially disposed streamlines for various collapsing geometries and for causing the streamlines to follow paths which are approximately equal in length. The invention further includes an apparatus for defining the streamline paths so as to cause them to be approximately equal in length.
Accordingly, it is a primary object of the present invention to provide a method and apparatus for processing blown film in which collapsing induced machine direction stresses at the frost band terminus are approximately equalized and thereby minimized.
It is another object of the present invention to provide a method and apparatus for processing blown film in which streamline lengths are approximately equalized so as to minimize collapsing induced machine direction stresses.
It is yet another object of the present invention to provide a method for processing blown film which includes approximating collapsing induced machine direction stresses which exist at the frost band terminus and causing the differential elements of the solidified film to follow streamline paths which are approximately equal in length so as to reduce the collapsing induced stresses to nearly zero.
Yet another object of the present invention is to provide methods for processing blown film which include approximating the streamline reference path lengths in a variety of collapsing geometries and causing the differential elements of the solidified film to follow streamline paths which are approximately equal to the length of the longest solidified streamline.
It is another object of this invention to provide methods and apparatuses for use with various collapsing geometries to minimize gauge variations and other deleterious quality affects as described herein by causing the differential elements of the solidified film to follow paths which are approximately equal in length.
Additional objects, advantages and other novel features of the invention will be set forth in part in the description that follows and in part will become apparent to those skilled in the art upon examination of the following or may be learned with the practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
To achieve the foregoing and other objects, and in accordance with the purposes of the present invention as described herein, there is provided a method for processing a continuous blown film which includes the steps of advancing the film between first and second locations which define a tension isolated region, the film including an orientation region with a terminus, collapsing the film to a layflat line within a collapsing region, the collapsing being effected by a collapsing geometry boundary defined by a collapsing geometry, and causing a plurality of differential elements of the film to follow respective streamline paths between the terminus and the second location which are approximately equal in length to the path of the longest streamline between the terminus and the second location.
In accordance with a further aspect of the present invention, the method includes the step of approximating the lengths of at least a segment of the respective reference paths of a plurality of streamlines within the tension isolated region.
In yet another aspect of the invention, the segments of the respective reference paths lie entirely within the collapsing region.
In a still further aspect of the present invention, the entrance of the collapsing region defines an entrance plane and the streamlines are assumed to enter the collapsing region generally perpendicular to the entrance plane.
According to a still further aspect of the invention, the cross-sectional shape of the film at the entrance of the collapsing region is assumed to be generally circular.
In yet another aspect of the invention, the segment of each respective streamline which extends between its respective contact point with the collapsing geometry boundary and the layflat line is assumed to lie in respective planes which are generally perpendicular to the layflat line.
According to a further aspect of the invention, the length of the lateral cross-sectional perimeter of the film is assumed to be constant within the collapsing region.
In yet another aspect of the invention, the lateral cross-sectional shape of the film within the collapsing region is assumed to be obround.
In accordance with yet another aspect of the invention, the lateral cross-sectional shape of the film within the collapsing region is assumed to be oblate obround.
In yet another aspect of the invention, the lengths of the segments of the reference paths of each streamline between the entrance and the respective streamline contact point with the collapsing geometry boundary is assumed to be equal to the length of a straight line between the intersection of the respective streamline with the entrance and the respective streamline contact point.
In accordance with a further aspect of the present invention, the reference streamline lengths between the terminus and the collapsing region entrance are assumed to be equal.
According to further aspects of the present invention, the collapsing of the film is effected by specifically identified collapsers, with collapser dependent formulas provided for approximating the lengths of various segments of the reference paths of the streamlines.
In accordance to another aspect of the present invention, the lateral cross-sectional shape of the film within the collapsing region is assumed to be elliptical for a spreader collapser.
In a still further aspect of the invention, a method for processing a continuous blown film is provided which includes the steps of advancing the film between first and second locations which define a tension isolated region, the film including an orientation region with a terminus, collapsing the film to at least one unwrap line within a collapsing region located within the tension isolated region, the collapsing being effected through an unwrap flattening process, and causing a plurality of differential elements of the film to follow respective streamline paths between the terminus and the second location which are approximately equal in length to the length of the longest streamline path between the terminus and the second location.
In accordance with another aspect of the present invention, the unwrap line is assumed to be offset from the central axis of the film.
According to a still further aspect of the present invention, a method of processing a continuous blown film is provided which includes steps of advancing the film between first and second locations which define a tension isolated region, the film including an orientation region within the tension isolated region, changing the cross-section shape of the film within a collapsing region located within the tension isolated region, and causing a plurality of differential elements of the film to follow respective paths between the terminus and the second location such that shape change induced machine direction stresses within the film at the terminus of the orientation region are approximately equal.
In yet a further aspect of the present invention, the shape change induced machine direction stresses at the terminus of the orientation region are approximately zero.
According to a still further aspect of the invention, specific formulas are provided for the approximation of stresses and reference stresses existent in the film.
In a still further aspect of the invention, an apparatus for minimizing the shape change induced stresses in a blown tube film includes a collapsing geometry structure for modifying the film shape between generally circular and generally layflat configurations as the film is advanced between first and second locations, the collapsing structure defining a collapsing geometry boundary adapted to contact variable segmental portions of the film, and being operative to cause advancing portions of the film that are in contact with the collapsing geometry boundary to follow predetermined streamline paths, and further including means for causing streamlines in those portions of the film which are not in contact with the collapsing geometry boundary to follow respective paths such that the lengths of the respective paths between the orientation region terminus and the second location are approximately equal.
In yet another aspect of the invention, an apparatus for causing streamlines of a continuously moving blown tube film to follow paths which are approximately equal in length is provided which includes a plurality of spaced apart internal film restricting surfaces and a respective external film restricting surface aligned between adjacent internal film restricting surfaces, the external film restricting surface being operable to cause the film to follow a serpentine path between the internal film restricting surfaces.
In accordance with a further aspect of the present invention, the external film restricting surface is movable between a first position at which it does not engage the film and a second position at which it does engage the film.
According to a still further aspect of the present invention, the internal film restricting surfaces are shaped complementary to the film.
According to yet another aspect of the present invention, the internal film restricting surfaces are disposed within the tube region.
According to another aspect of the present invention, the internal film restricting surfaces are. disposed within the collapsing region.
In accordance to yet another aspect of this invention, the internal film restricting surfaces are shaped complementary to the sail sections of the film.
In still another aspect of the invention, the external restricting surface cooperates with the internal restricting surfaces to cause the streamlines to follow respective serpentine paths whose lengths vary in dependence upon the circumferential position of the respective streamlines.
According to another aspect of the present invention, a device for defining the paths of streamlines of a continuously moving blown film is provided which includes a plurality of spaced apart idler rolls disposed adjacent the film in a layflat region and at least one streamline path defining roll disposed on the opposite side of the film from the idler rolls and aligned with the gap therebetween, the path defining roll being operable to engage the film so as to cause differential elements of the film to follow respective serpentine paths between adjacent idler rolls, the respective lengths of the respective serpentine paths varying in dependence upon the circumferential position of the differential elements.
In still a further aspect of the present invention, the path defining roll is movable between a first position at which it does not engage the film and a second position at which does engage the film.
In accordance with another aspect of the present invention, the path defining roll comprises at least two rolls whose positions relative to each other may be varied.
Still other objects of the present invention will become apparent to those skilled in this art from the following description wherein there is shown and described a preferred embodiment of this invention, simply by way of illustration, of one of the best modes contemplated for carrying out the invention. As will be realized, the invention is capable of other different embodiments, and its several details are capable of modification in various, obvious aspects all without departing from the invention. Accordingly, the drawings and descriptions will be regarded as illustrative in nature and not as restrictive.